The aforementioned technical paper describes a microprocessor-based automatic draft control system for a three-point hitch. A tillage implement is attached to the hitch and the hitch is mounted at the rear of a tractor having pneumatic tires. A single quadrant lever, located in the tractor cab, is manually moved by the operator to generate position/draft commands which cause raising or lowering of the hitch. Draft sensors sense the draft on the hitch and a position sensor senses the position of the hitch, the sensors producing feedback signals to the microprocessor. The microprocessor responds to the position/draft commands and the feedback signals from the sensors by generating output signals for adjusting the position of the hitch by selectively energizing raise and lower solenoids associated with an electrohydraulic hitch lift valve.
The system described in the technical paper and the system disclosed in U.S. Pat. No. 4,846,283 are provided with a manually operable drop rate potentiometer whereby an operator may adjust the maximum velocity at which a hitch may be lowered. However, these systems employ an open loop rate control. The drop rate signal derived from the drop rate potentiometer modifies the hitch movement command signal which is used to control the movement of the hitch. By limiting the magnitude of the hitch movement command signal the rate of hitch drop is limited. This arrangement does not take into account variable factors such as implement weight, system temperature, valve characteristics, etc. which affect system performance. Thus, these systems do not provide the accurate velocity control necessary when an implement is lowered as it enters a field from a headland. Without accurate velocity control the operator cannot maintain an even headland.
Accurate velocity control is particularly important when an operator is using a manually actuated switch to adjust the position of the hitch. As described in the aforementioned technical paper, switches may be provided on the rear fenders of a tractor so as to be accessible to the operator as he attempts to align the hitch and implement for attachment or removal of the implement.